CAT or O2 Sensor?

[SCZ]

I have an '03 Silverado Z71 and the check engine light came on late last night. With the lack of time that I had today I ran by Auto Zone to have them run the code to see what it may be. The fellow told me either that it was either the Cat or the O2 Sensor. Don't think that the guy knew that there were 4 sensors and 2 cats.

 

Usually on these Trucks which usally goes out first the front sensors or the Cats? What is the life span of the cats? And are the Universal replacement sensors Safe?

[dougmac]

Did you get the code? The code will tell you specifically which sensor is having the problem.

[SCZ]

Unfortunately I did not. I forgot to ask and the fellow didn't act like he wanted to be at work on a Monday.